Method and apparatus for delaying a ventricular tachycardia therapy

ABSTRACT

A device and method to detect slow ventricular tachycardia, deliver anti-tachycardia pacing therapies, and delay a scheduled shock therapy if the ventricular tachycardia is not terminated or accelerated. Preferably, a shock therapy is delayed after verifying hemodynamic stability based on a hemodynamic sensor. After a shock is delayed, the device operates in a high alert mode for redetecting an accelerated tachycardia. Anti-tachycardia pacing therapies are repeated during the shock delay. A number of conditions can trigger delivery of the delayed shock therapy including a specified period of elapsed time; determination that the patient is likely to be asleep; detection of myocardial ischemia; detection of compromised hemodynamics, or detection of a substantially prone position or sudden change in position. A delayed shock therapy may be triggered by the patient and repeated delivery of painful shock therapies in patients that are not seriously compromised by a recurring, slow ventricular tachycardia is avoided.

RELATED APPLICATION

The following is a divisional application of and claims priority andother benefits from U.S. patent application Ser. No. 10/134,352, filedApr. 26, 2002, entitled “METHOD AND APPARATUS FOR DELAYING A VENTRICULARTACHYCARDIA THERAPY,” incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an implantable cardiac stimulationdevice capable of delivering anti-tachycardia therapy and morespecifically a device and method for delaying shock therapies when adetected slow ventricular tachycardia is determined to be stable.

BACKGROUND

Implantable medical devices are available for treating cardiacarrhythmias by delivering electrical shock therapy for cardioverting ordefibrillating the heart in addition to cardiac pacing. Such a device,commonly known as an implantable cardioverter defibrillator or “ICD”,senses a patient's heart rhythm and classifies the rhythm according to anumber of rate zones in order to detect episodes of tachycardia orfibrillation. Single chamber devices are available for treating eitheratrial arrhythmias or ventricular arrhythmias, and dual chamber devicesare available for treating both atrial and ventricular arrhythmias. Ratezone classifications typically include normal sinus rhythm, tachycardia,and fibrillation.

Upon detecting an abnormal rhythm, the ICD delivers an appropriatetherapy. Cardiac pacing is delivered in response to the absence ofsensed intrinsic depolarizations, referred to as P-waves in the atriumand R-waves in the ventricle. Ventricular fibrillation (VF) is a seriouslife-threatening condition and is normally treated by immediatelydelivering high-energy shock therapy. Termination of VF is normallyreferred to as “defibrillation.”

In response to tachycardia detection, a number of tiered therapies maybe delivered beginning with anti-tachycardia pacing therapies andescalating to more aggressive shock therapies until the tachycardia isterminated. Termination of a tachycardia is commonly referred to as“cardioversion.” In modern implantable cardioverter defibrillators, thephysician programs the particular therapies into the device ahead oftime, and a menu of therapies is typically provided. For example, oninitial detection of an atrial or ventricular tachycardia, ananti-tachycardia pacing therapy may be selected and delivered to thechamber, in which the tachycardia is diagnosed or to both chambers. Onredetection of tachycardia, a more aggressive anti-tachycardia pacingtherapy may be scheduled. If repeated attempts at anti-tachycardiapacing therapies fail, a higher energy cardioversion pulse may beselected. Therapies for tachycardia termination may also vary with therate of the detected tachycardia, with the therapies increasing inaggressiveness as the rate of the detected tachycardia increases. Forexample, fewer attempts at anti-tachycardia pacing may be undertakenprior to delivery of cardioversion pulses if the rate of the detectedtachycardia is above a preset threshold. For an overview of tachycardiadetection and treatment therapies reference is made to U.S. Pat. No.5,545,186 issued to Olson et al.

Ventricular tachycardia (VT) may be debilitating, but is not necessarilyan immediately life-threatening situation. Cardiac output tends to becompromised due to the disorganized contraction of the myocardial tissueresulting in a patient feeling weak, dizzy or even fainting. Ventriculartachycardia may, however, degenerate into a more unstable heart rhythm,leading to ventricular fibrillation. Therefore in most cases, it isdesirable to immediately treat a detected VT, either withanti-tachycardia pacing therapies or cardioversion shocks. Because VTcan often be terminated by known anti-tachycardia pacing therapies,these therapies are generally delivered first, because they are lesspainful to the patient, then followed by high-energy shock therapy ifnecessary.

However, in some cases, a patient may be diagnosed with a recurrentslow-rate ventricular tachycardia that is not associated with symptomsof hemodynamic compromise. When a recurrent VT is repeatedly detected byan ICD device, the patient will normally undergo a preset menu of tieredtherapies, which may conclude with shock delivery in order to terminatethe VT. Therefore, a patient having a recurrent VT may be repeatedlysubjected to painful shock therapies. In a patient having recurrent, buthemodynamically stable, slow VT, such repeated shock therapy may beundesirable since the condition is not immediately life-threatening andnot expected to deteriorate into a more serious tachycardia. Animplantable cardioverter defibrillator device capable of delaying orsuspending a high-energy shock therapy in response to detecting astable, low-rate ventricular tachycardia is therefore needed.

SUMMARY

The present invention addresses, inter alia, this problem of repeatedshock delivery in patients having stable, low rate ventriculartachycardia. Aspects of the present invention include delaying thedelivery of painful shock therapy in patients having recurrent slow VT,particularly when the patient is determined not to be hemodynamicallycompromised. Further aspects of the present invention includecontrolling the time of shock therapy delivery, so that non-criticalshocking therapies are delivered at a time that the patient is not atfurther risk of injury or pain, and potentially averting the need forshock therapy by allowing continued attempts of anti-tachycardia pacingtherapies to terminate the abnormal rhythm prior to delivering a delayedshock therapy.

These aspects are realized by providing an implantable medical devicefor delivering anti-tachyarrhythmia and defibrillation therapies to theheart, and specifically to the ventricles, in the form of pacing orshocking pulses and an associated method for discriminating between alow-rate or stable form of ventricular tachycardia and other, higherrate or unstable forms of ventricular tachycardia. An associated methodincludes first delivering anti-tachycardia pacing therapies when a slow,stable ventricular tachycardia is detected and delaying a programmedshock therapy.

The present invention includes a “high alert” mode executed during theperiod of delayed shock therapy to allow prompt therapy delivery shouldthe heart rhythm accelerate or should other conditions arise indicatinga need for shock therapy. During the high alert mode, less stringentredetection criteria is used than during normal device operation forarrhythmia detection. For example, the high alert redetection criteriamay require fewer intervals within a VT or VF zone to allow for morerapid detection and therapy response.

The methods included in the present invention are enhanced byimplementing a sensor of hemodynamic function. Detection of a VT withconfirmation of stable hemodynamic function justifies delaying a shocktherapy to a later time. Detection of a VT with decreased hemodynamicfunction, however, indicates a need for more immediate therapy. Adelayed shock therapy is immediately delivered if compromisedhemodynamic function is detected. During the delay period,anti-tachycardia pacing therapies are repeated in an attempt to restorethe heart to normal sinus rhythm and avert the need for any shockingtherapy.

Another feature of the present invention includes the programmableselection of conditions under which a delayed shock therapy is deliveredin order to regain hemodynamic support or avoid the development ofmyocardial ischemia. For example, a delayed shock may be delivered aftera specified period of elapsed time or after determining that the patientis likely to be resting or asleep. A delayed shock may be delivered upondetection of myocardial ischemia or detection of a substantially proneposition or sudden change in position indicating that the patient mayhave fallen due to compromised hemodynamic output. A patient orphysician issued command may also trigger a delayed shock therapy.

One aspect of the present invention is the ability to choose betweenconventional treatment modalities of escalating therapies or delayingmore aggressive shock therapies in patients who are diagnosed withhemodynamically stable ventricular tachycardia. Repeated delivery ofpainful shock therapies in patients that are not seriously compromisedby a recurring, stable, low-rate tachyarrhythmia is avoided. By avoidingfrequently repeated shock therapies, the life-expectancy of thebattery-powered implantable device is extended, and battery charge isreserved for more serious, life-threatening occurrences of tachycardiaor fibrillation. Furthermore, the present invention allows the shocktherapy to be delivered at a controlled time, for example, after thepatient has had time to seek medical attention or at a time when thepatient is not at risk of further injury, such as while driving a car.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an implantable cardiac stimulation devicecapable of pacemaking, cardioversion, and defibrillation and incommunication with a patient's heart via three stimulation and sensingleads;

FIG. 2 is a high-level, functional, block diagram of the implantablepacemaker cardioverter defibrillator shown in FIG. 1;

FIG. 3 is a flow chart illustrating a method performed by the deviceshown in FIG. 2 for delaying a ventricular tachycardia therapy accordingto one embodiment of the present invention;

FIG. 4 is a flow chart illustrating a method for delaying a ventriculartachycardia therapy according to another embodiment of the presentinvention that includes hemodynamic monitoring;

FIG. 5 is a flow chart illustrating the operations performed by thedevice shown in FIG. 2 during a period of delayed shock therapy;

FIG. 6 is a flow chart illustrating the operations performed by thedevice shown in FIG. 2 for detecting conditions that will trigger thedelivery of a delayed shock therapy, and

FIG. 7 is an illustration of a patient activator that may be used by apatient to trigger the delivery of a delayed therapy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a system and method for delaying shocktherapies upon detection of a slow, stable ventricular tachycardia. Themethods included in the present invention are preferably incorporated inan implantable cardiac stimulation device capable of deliveringanti-arrhythmia therapies, such as the implantable cardioverterdefibrillator, or “ICD,” shown in FIG. 1.

The ICD 10 is shown coupled to a patient's heart by way of three leads6, 15, and 16. A connector block 12 receives the proximal end of a rightventricular lead 16, a right atrial lead 15 and a coronary sinus lead 6,used for positioning electrodes for sensing and stimulation in three orfour heart chambers. In FIG. 1, the right ventricular lead 16 ispositioned such that its distal end is in the vicinity of the rightventricle for sensing right ventricular cardiac signals and deliveringpacing or shocking pulses in the right ventricle. For these purposes,right ventricular lead 16 is equipped with a ring electrode 24, anextendable helix electrode 26 mounted retractably within an electrodehead 28, and a coil electrode 20, each of which are connected to aninsulated conductor within the body of lead 16. The proximal end of theinsulated conductors are coupled to corresponding connectors carried bybifurcated connector 14 at the proximal end of lead 16 for providingelectrical connection to the ICD 10.

The right atrial lead 15 is positioned such that its distal end is inthe vicinity of the right atrium and the superior vena cava. Lead 15 isequipped with a ring electrode 21 and an extendable helix electrode 17,mounted retractably within electrode head 19, for sensing and pacing inthe right atrium. Lead 15 is further equipped with a coil electrode 23for delivering high-energy shock therapy. The ring electrode 21, thehelix electrode 17 and the coil electrode 23 are each connected to aninsulated conductor within the body of the right atrial lead 15. Eachinsulated conductor is coupled at its proximal end to a connectorcarried by bifurcated connector 13.

The coronary sinus lead 6 is advanced within the vasculature of the leftside of the heart via the coronary sinus and great cardiac vein. Thecoronary sinus lead 6 is shown in the embodiment of FIG. 1 as having adefibrillation coil electrode 8 that may be used in combination witheither the coil electrode 20 or the coil electrode 23 for deliveringelectrical shocks for cardioversion and defibrillation therapies. Inother embodiments, coronary sinus lead 6 may also be equipped with adistal tip electrode and ring electrode for pacing and sensing functionsin the left chambers of the heart. The coil electrode 8 is coupled to aninsulated conductor within the body of lead 6, which provides connectionto the proximal connector 4.

The electrodes 17 and 21 or 24 and 26 may be used as bipolar pairs,commonly referred to as a “tip-to-ring” configuration, or individuallyin a unipolar configuration with the device housing 11 serving as theindifferent electrode, commonly referred to as the “can” or “case”electrode. The device housing 11 may also serve as a subcutaneousdefibrillation electrode in combination with one or more of the coilelectrodes 8, 20 or 23 for defibrillation of the atria or ventricles. Itis recognized that alternate lead systems may be substituted for thethree lead system illustrated in FIG. 1.

Although three or four-chamber pacing, cardioversion and defibrillationcapacity is not necessary for practicing the invention, and indeeddetection of slow ventricular tachycardia can be determined by sensingonly signals derived from the right ventricle, a multi-chamber system isillustrated so as to indicate the scope of the invention. It isunderstood that the invention may normally be practiced with amulti-chamber, dual chamber, or single chamber device.

A functional schematic diagram of the ICD 10 is shown in FIG. 2. Thisdiagram should be taken as exemplary of the type of device in which theinvention may be embodied and not as limiting. The disclosed embodimentshown in FIG. 2 is a microprocessor-controlled device, but the methodsof the present invention may also be practiced in other types of devicessuch as those employing dedicated digital circuitry.

With regard to the electrode system illustrated in FIG. 1, the ICD 10 isprovided with a number of connection terminals for achieving electricalconnection to the leads 6, 15, and 16 and their respective electrodes.The connection terminal 311 provides electrical connection to thehousing 11 for use as the indifferent electrode during unipolarstimulation or sensing. The connection terminals 320, 310, and 318provide electrical connection to coil electrodes 20, 8 and 23respectively. Each of these connection terminals 311, 320, 310, and 318are coupled to the high voltage output circuit 234 to facilitate thedelivery of high energy shocking pulses to the heart using one or moreof the coil electrodes 8, 20, and 23 and optionally the housing 11.

The connection terminals 317 and 321 provide electrical connection tothe helix electrode 17 and the ring electrode 21 positioned in the rightatrium. The connection terminals 317 and 321 are further coupled to anatrial sense amplifier 204 for sensing atrial signals such as P-waves.The connection terminals 326 and 324 provide electrical connection tothe helix electrode 26 and the ring electrode 24 positioned in the rightventricle. The connection terminals 326 and 324 are further coupled to aventricular sense amplifier 200 for sensing ventricular signals.

The atrial sense amplifier 204 and the ventricular sense amplifier 200preferably take the form of automatic gain controlled amplifiers withadjustable sensing thresholds. The general operation of the ventricularsense amplifier 200 and the atrial sense amplifier 204 may correspond tothat disclosed in U.S. Pat. No. 5,117,824, by Keimel, et al.,incorporated herein by reference in its entirety. Whenever a signalreceived by atrial sense amplifier 204 exceeds an atrial sensingthreshold, a signal is generated on the P-out signal line 206. Whenevera signal received by the ventricular sense amplifier 200 exceeds aventricular sensing threshold, a signal is generated on the R-out signalline 202.

Switch matrix 208 is used to select which of the available electrodesare coupled to a wide band amplifier 210 for use in digital signalanalysis. Selection of the electrodes is controlled by themicroprocessor 224 via data/address bus 218. The selected electrodeconfiguration may be varied as desired for the various sensing, pacing,cardioversion and defibrillation functions of the ICD 10. Signals fromthe electrodes selected for coupling to bandpass amplifier 210 areprovided to multiplexer 220, and thereafter converted to multi-bitdigital signals by A/D converter 222, for storage in random accessmemory 226 under control of direct memory access circuit 228.Microprocessor 224 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 226 torecognize and classify the patient's heart rhythm employing any of thenumerous signal processing methodologies known in the art. Atachyarrhythmia recognition mechanism is described in U.S. Pat. No.5,987,356, issued to DeGroot and in the previously referenced U.S. Pat.No. 5,545,186 issued to Olson et al, both of which patents areincorporated herein by reference in their entirety.

The telemetry circuit 330 receives downlink telemetry from and sendsuplink telemetry to an external programmer, as is conventional inimplantable anti-arrhythmia devices, by means of an antenna 332. Data tobe uplinked to the programmer and control signals for the telemetrycircuit are provided by microprocessor 224 via address/data bus 218.Received telemetry is provided to microprocessor 224 via multiplexer220. Numerous types of telemetry systems known for use in implantabledevices may be used. The telemetry circuit 330 is also used forcommunication with a patient activator in one embodiment of the presentinvention.

In a preferred embodiment, the device 10 is equipped with a sensor 344and sensor processing circuitry 342. Depending on the type of sensorused, the sensor 344 may be located within the device housing 11 orexternal to the device housing 11 but implanted within the body of thepatient. In one embodiment, the sensor 344 is used for determining thehemodynamic status of the patient. The sensor 344 may therefore be apressure sensor for sensing a patient's blood pressure within the heartchambers or vasculature, an impedance sensor for sensing thoracicimpedance, a blood oxygen sensor, a blood pH sensor, or any knownsensor, or combination of sensors, capable of providing a signal thatcan be correlated to a patient's hemodynamic status. Pressure sensorsthat may be implemented with the ICD 10 for monitoring hemodynamicstatus are generally described in U.S. Pat. No. 6,171,252 to Roberts,and U.S. Pat. No. 6,221,024 to Miesel, both patents incorporated hereinby reference in their entirety. In accordance with one embodiment of thepresent invention, signals received by the sensor processing circuit 342from the sensor 344 can be analyzed for detecting a change in apatient's hemodynamic status particularly during a detected slow VT, aswill be described in greater detail below.

In an alternative embodiment, the sensor 344 takes the form of apositional sensor for determining the posture of the patient. A methodand apparatus for determining the physical posture of a patient's bodyis disclosed in U.S. Pat. No. 6,044,297 to Sheldon et al., incorporatedherein by reference in its entirety. In one embodiment of the presentinvention, detection of a substantially prone position, or preferably asudden change in position occurring after a VT detection, is used todetermine that a patient may have fallen as a result of compromisedhemodynamic status. In another embodiment of the present invention, aposture sensor may be used in combination with the time of day or anactivity sensor for determining when the patient is asleep. Reference ismade to U.S. Pat. No. 5,233,984 issued to Thompson, U.S. Pat. No.5,593,431 issued to Sheldon, and U.S. Pat. No. 5,630,834 issued toBardy, all of which are incorporated herein by reference in theirentirety. Therefore, in certain embodiments, sensor 344 may represent acombination of sensors such as a pressure sensor, an activity sensor,and a posture sensor such that a change in hemodynamic status and/orsleep and/or posture may be detected by microprocessor 224

The remainder of the circuitry illustrated in FIG. 2 is an exemplaryembodiment of circuitry dedicated to providing cardiac pacing,cardioversion and defibrillation therapies. The pacer timing and controlcircuitry 212 includes programmable digital counters which control thebasic time intervals associated with various single, dual ormulti-chamber pacing modes or anti-tachycardia pacing therapiesdelivered in the atria or ventricles. Pacer circuitry 212 alsodetermines the amplitude of the cardiac pacing pulses under the controlof microprocessor 224.

During pacing, escape interval counters within pacer timing and controlcircuitry 212 are reset upon sensing of R-waves or P-waves as indicatedby signals on lines 202 and 206, respectively. In accordance with theselected mode of pacing, pacing pulses are generated by atrial paceroutput circuit 214 and ventricular pacer output circuit 216. The paceroutput circuits 214 and 216 are coupled to the desired electrodes forpacing via switch matrix 208. The escape interval counters are resetupon generation of pacing pulses, and thereby control the basic timingof cardiac pacing functions, including anti-tachycardia pacing.

The durations of the escape intervals are determined by microprocessor224 via data/address bus 218. The value of the count present in theescape interval counters when reset by sensed R-waves or P-waves can beused to measure R-R intervals and P-P intervals for detecting theoccurrence of a variety of arrhythmias.

The microprocessor 224 includes associated ROM in which stored programscontrolling the operation of the microprocessor 224 reside. A portion ofthe memory 226 may be configured as a number of recirculating bufferscapable of holding a series of measured intervals for analysis by themicroprocessor 224 for predicting or diagnosing an arrhythmia.

In response to the detection of tachycardia, anti-tachycardia pacingtherapy can be delivered by loading a regimen from microcontroller 224into the pacer timing and control circuitry 212 according to the type oftachycardia detected. In the event that higher voltage cardioversion ordefibrillation pulses are required, microprocessor 224 activates thecardioversion and defibrillation control circuitry 230 to initiatecharging of the high voltage capacitors 246 and 248 via charging circuit236 under the control of high voltage charging control line 240. Thevoltage on the high voltage capacitors is monitored via a voltagecapacitor (VCAP) line 244, which is passed through the multiplexer 220.When the voltage reaches a predetermined value set by microprocessor224, a logic signal is generated on the capacitor full (CF) line 254,terminating charging. The defibrillation or cardioversion pulse isdelivered to the heart under the control of the pacer timing and controlcircuitry 212 by an output circuit 234 via a control bus 238. The outputcircuit 234 determines the electrodes used for delivering thecardioversion or defibrillation pulse and the pulse wave shape.

In one embodiment, the ICD 10 may be equipped with a patientnotification system 150 used to notify the patient that shock therapy isbeing withheld. Any patient notification method known in the art may beused, such as generating perceivable twitch stimulation or an audiblesound. A patient notification system may include an audio transducerthat emits audible sounds including voiced statements or musical tonesstored in analog memory and correlated to a programming or interrogationoperating algorithm or to a warning trigger event as generally describedin U.S. Pat. No. 6,067,473 issued to Greeninger et al., incorporatedherein by reference in its entirety.

In FIG. 3, a flow diagram is shown illustrating operations included inone embodiment of the present invention for delaying a programmed shocktherapy in response to detecting a slow ventricular tachycardia. Thesteps illustrated in FIG. 3 are preferably carried out under the controlof microprocessor 224. The method 400 is preferably enabled or disabledby a telemetered command delivered by a physician using an externalprogrammer in communication with telemetry circuit 330. Upon enablingthe method 400, the external programmer may display a message warningthe physician that the presently programmed selection may result in awithholding of therapies and requesting confirmation of this selection.

When enabled, the method 400 begins at step 405 whenever microprocessor224 detects a ventricular tachycardia based on VT detection criteria. VTdetection criteria are typically defined by a programmed number ofconsecutively measured R-R intervals falling within a VT detection zone.At step 410, the tachycardia is classified as a slow VT or a fast VTbased on the detected cycle length. Typically, a VT has a cycle lengthbetween 250 and 500 ms. A “slow” VT may generally be characterized by acycle length between 450 to 500 ms and may be even longer, and a “fast”VT may generally be characterized by a cycle length of 250 to 300 ms.The tachycardia rate detection zones and detection criteria arepreferably programmable parameters that allow selection of detectioncriteria to be tailored according to individual patient need.

If the initial VT detection is not a slow VT, as determined at decisionstep 410, anti-tachycardia therapies are delivered at step 430 accordingto programmed regimens, which can include anti-tachycardia pacingtherapies and cardioversion shocks. If the initial VT detection isdetermined to be a slow VT at decision step 410, any anti-tachycardiapacing therapies programmed to be delivered in response to VT detectionare delivered at step 420. At decision step 425, the microprocessor 224determines if termination of the slow VT is detected. Termination isgenerally defined as a given number of consecutively sensed R-Rintervals that are greater in length than the programmed VT detectioninterval. If anti-tachycardia pacing therapies have successfullyterminated the slow VT, the method 400 is terminated at step 435.

If the microprocessor 224 does not detect termination at step 425 andcontinues to redetect slow VT at decision step 440 even after theprogrammed regimen of anti-tachycardia pacing therapies has beenexhausted, the delivery of any programmed shock therapies is delayed atstep 445. If, however, the slow VT has accelerated, as determined atdecision step 440, then appropriate therapies are delivered at step 430in response to the detected rhythm, either a fast VT or VF. Anaccelerated VT is generally detected when the sensed cycle length hasshortened by a given interval, for example 60 ms, compared to theaverage cycle length before redetection.

If the slow VT did not accelerate, resulting in a delayed shock therapyat step 445, an optional patient notification signal may be generated bynotification system 150 at step 447 to alert the patient that a therapyis being withheld. Such patient notification allows the patient to seekmedical attention if desired. Notifying the patient that a shock therapyhas been delayed also allows the patient to retire to a controlledsetting for self-initiating a delayed shock therapy, such as at homeresting rather than in a work place or driving a car.

After a shock therapy is delayed, the ICD 10 operates in a high alertmode as shown by step 500. The operations of the ICD 10 during the highalert mode 500 will be described later in greater detail with referenceto the flow chart shown in FIG. 5.

FIG. 4 shows a flow chart summarizing a method 600 for delaying a shocktherapy that includes monitoring a patient's hemodynamic status. In apreferred embodiment, the patient's hemodynamic status is a conditionfor delaying or delivering a shock therapy. In this embodiment, thesensor 344 provides a signal related to the hemodynamic status of thepatient so that even during a slow VT a shock therapy is still deliveredif the patient's hemodynamic status is compromised and pacing therapieswere not successful in terminating the VT.

In one method of operation, a hemodynamic threshold level may bepredetermined or programmable and stored in memory 226 for defining acompromised hemodynamic state. If the sensor processing circuit 342determines that the hemodynamic status of the patient has deterioratedbeyond the threshold level, a shock therapy is immediately initiated. Inother methods of operation, a running average of a hemodynamic parametermay be determined and a change based on a percentage or the standarddeviation of the average may be used to a detect a compromisedhemodynamic state.

In FIG. 4, the steps 405 through 440 are performed exactly as describedpreviously with reference to method 400 shown in FIG. 3. When a slow VTis detected (step 410), programmed anti-tachycardia pacing therapies aredelivered (step 420). If termination is not detected (step 425) and theslow VT is sustained (step 440), the microprocessor 224 determines atstep 605 if the patient is hemodynamically stable based on sensorprocessing 342 before delaying the shock therapy at step 445. At anytime, if the detected rhythm is a fast VT or VF (steps 410 or 440), allprogrammed therapies, including shocks, are delivered (step 430). If ashock therapy is delayed (step 445) based on a slow VT detection andverification of stable hemodynamics at step 605, an optional patientnotification may be delivered (step 447), and the ICD 10 beginsoperating in the high alert mode 500.

Operations performed during the high alert mode are summarized by theflow chart shown in FIG. 5. The microprocessor 224 executes the method500 to control when and if a delayed shock therapy will be delivered andto detect and respond quickly to any worsening of the heart rhythm. Atdecision step 505, microprocessor 224 continues to monitor fortermination of the slow VT, which may still occur spontaneously duringthe shock delay period. If spontaneous termination is detected, thedelayed shock therapy is canceled at step 540. In one embodiment, theoptional patient notification system 150 may generate a second sound orvoiced statement at step 550 to notify the patient that a withheld shocktherapy is no longer needed and will not be delivered. The high alertmode 500 is then terminated at step 560.

If, however, termination is not detected, the ICD 10 continues toredetect the slow VT and remains on “high-alert” for detection of fastVT or VF at decision step 510. This high-alert mode allows the ICD 10 todetect a fast VT or VF more quickly than during the normal detectionmode by using less stringent detection criteria. The detection criteriautilized during the high-alert mode may be similar to criteria used forredetection during a VT or VF episode. During a VT or VF episode, thenumber of intervals required for redetecting VT or VF after delivering atherapy is commonly programmed to be smaller than the number ofintervals required for an initial VT detection. For example, twelveconsecutive intervals shorter than the specified VT interval may berequired to initially detect VT, whereas only eight intervals might berequired to redetect VT after a therapy has been delivered. Similarly,fewer cardiac cycles may be required for redetecting VT or VF during thehigh-alert mode included in the present invention than for the initialVT/VF detection.

If the slow VT accelerates or becomes unstable, as determined atdecision step 510 according to the less stringent high-alert detectioncriteria, programmed therapies, including cardioversion ordefibrillation shocks, are immediately delivered at step 545 accordingto the detected rhythm. However, if the slow VT is sustained at decisionstep 510, the programmed anti-tachycardia pacing therapies that weredelivered previously (at step 420, FIG. 4) are repeated at step 520, ina further attempt to terminate the slow VT without delivering acardioversion shock. The pacing therapies may be repeated after apredetermined time interval, for example every five minutes or everyhour. In one embodiment, pacing therapies may be re-delivered uponsensing a change in the heart rhythm that is thought to increase thelikelihood of a successful termination. Reference is made to pendingU.S. patent application Ser. No. 10/034,060 entitled “AutomatedReapplication of Atrial Pacing Therapies”, to Hess et al. filed on Dec.20, 2001, incorporated herein by reference in its entirety. It isfurther recognized that a previously delivered regimen of pacingtherapies may be repeated at step 520 or different regimens may bedelivered in an alternating or cyclical fashion. Pacing therapies thatare considered first tier therapies, in that they are not likely toinduce VF, may be preferred over second tier pacing therapies that areknown to be more likely to induce VF.

At decision step 525, the microprocessor 224 continues to monitor fortermination of the slow VT, which may occur either spontaneously or inresponse to a successful anti-tachycardia pacing therapy. If terminationis detected, the delayed shock therapy is cancelled at step 540, anoptional patient notification is generated at step 550 and the method500 is terminated at step 560.

If termination is not detected at decision step 525, the microprocessor224 determines if a set of predefined conditions for delivering adelayed shock therapy are met at decision step 530. If these conditionsare met, the delayed shock therapy is delivered at step 535. If theseconditions are not met, the method 500 returns to step 505 and continuesin the high alert mode, monitoring for termination or a worsening heartrhythm and repeating anti-tachycardia pacing therapies. This process,steps 505 through 530, continues until either termination is detected orthe conditions required for delivering a delayed shock therapy are met.

One or more conditions, other than an accelerated heart rhythm, may beset as prerequisites for delivering a delayed shock therapy. In oneembodiment, a required amount of time must elapse prior to deliveringthe delayed shock. Setting a predetermined amount of time until adelayed shock therapy is delivered and notifying the patient that atherapy is being withheld allows the patient to seek medical attentionor become situated in a safe, resting position prior to shock delivery.By delaying the shock a given amount of time, the slow VT mayspontaneously terminate during the delay period, or the repeatedanti-tachycardia pacing therapies may be successful. In either case, theshock therapy is averted, and the patient is spared from receiving apainful shock. The predetermined amount of elapsed time may be on theorder of hours but is preferably not longer than twenty-four hours. Asustained ventricular tachycardia, even if stable in the short term, caneventually lead to myocardial ischemia and symptoms of heart failure,making it undesirable to withhold VT therapy indefinitely.

In another embodiment, a required condition for delivery of a delayedshock therapy is detection of a prone position or a sudden change inposition. Patients may experience dizziness and even fainting due todecreased cardiac output during ventricular tachycardia. A positionsensor included in sensor 344 enables microprocessor 224 to detect asudden change in position as evidence that the patient has fallen due toinsufficient cardiac output. Detection of a sudden change in position bymicroprocessor 224 during a sustained slow VT, therefore, could triggerthe delivery of a delayed shock therapy.

In another embodiment, a condition for delivering a delayed shock may bea determination that the patient is likely to be asleep. Any method forsleep detection known in the art may be used. For example, a combinationof an activity sensor and a posture sensor may be used to detect a lowlevel of activity and a prone position as evidence that the patient islikely to be asleep.

One risk of sustained ventricular tachycardia is the development ofmyocardial ischemia. Therefore, in one embodiment, an ischemia detectionalgorithm may be included in the ICD 10 for monitoring for myocardialischemia during the high alert operating mode. Evidence of myocardialischemia can be obtained from the sensed myocardial electrogram (EGM).In particular, ST-segment deviations detected in the sensed EGM signalscan indicate myocardial ischemia. Any method for detecting myocardialischemia known in the art may be used. One method for myocardialischemia detection is described in U.S. Pat. No. 6,128,526 issued toStadler et al., incorporated herein by reference in its entirety. If acondition of myocardial ischemia is detected during the high alertoperation mode, the delayed shock therapy may immediately be delivered.

It is recognized that any of a number of conditions or combination ofconditions may be utilized for triggering a delayed shock therapy atstep 530 during the high alert mode (FIG. 5). The flow chart shown inFIG. 6 summarizes one example of a number of conditions that may betested for during step 530 of method 500 in order to determine if adelayed shock therapy needs to be delivered. At step 562, themicroprocessor 224 determines if the patient is hemodynamically stableaccording to output from sensor processing 342 based one or morehemodynamic sensor signals obtained from sensor 344. If hemodynamicinstability is detected, based on predetermined or programmablehemodynamic threshold settings, microprocessor 224 determines that theconditions for delivering a delayed shock therapy are met at step 572.The method 500 will then proceed to step 535 (FIG. 4) to deliver thedelayed shock.

If hemodynamic instability is not detected at step 562, themicroprocessor 224 screens for myocardial ischemia at step 564. Ifmyocardial ischemia is detected, based on predetermined or programmableischemia threshold settings, the conditions for delivering a delayedshock are met at step 572. If myocardial ischemia is not detected, themicroprocessor 224 next determines if the patient is asleep at decisionstep 568. If sleep is detected, the shock conditions are met at step572, and the delayed shock will be delivered. If none of the aboveconditions are met, the microprocessor 224 determines at step 570 if apredetermined delay time has elapsed in pacer timing and control 212. Ifthe time has elapsed, the delayed shock will be delivered. If the delaytime has not elapsed, the conditions for delivering a delayed shock arenot met, as indicated at step 574, and steps 562 through 570 will berepeated. It is recognized that although the operations included inmethod 530 shown in FIG. 6 are illustrated as discreet steps, monitoringfor the shock conditions may be performed continuously andsimultaneously throughout the high-alert mode 500. In addition, any ofthese or other shock conditions may be programmed to be enabled ordisabled by a clinician such that shock conditions can be tailored toindividual patient need.

In yet another embodiment, the delayed shock therapy may be triggered bya patient or physician issued command anytime during the delay period.Such a command may be issued using an external programmer incommunication with the ICD 10 via the telemetry circuitry 330. A commandmay also be given by a patient using a patient activator such as thetype of activator shown in FIG. 7. A patient activator 100 is typicallya hand-held device with a push button 102 that when depressed triggersthe delivery of a therapy if the activator is positioned withintelemetric communication distance from the implanted device. Theactivator 100 is generally battery-powered and provided with a batterycompartment 104. A speaker 112 is provided for broadcasting patientalert signals during telemetric communication with the ICD 10. Two,differently colored LEDs 116 are provided for signaling informationregarding the status of a patient-triggered therapy.

Alternative patient-initiated triggers for delivering a delayed shocktherapy may include a specific action by the patient. For example,breath-holding by the patient in an effort to brace himself for apending shock may be sensed by an impedance sensor included in ICD 10.In another example, tapping on the implanted ICD 10 by the patient maybe sensed by a piezoelectric crystal located within the ICD 10 housing.When a shock therapy is delayed, the patient may be alerted to the needfor a shock therapy by the patient notification system 150 within ICD10. The patient may then initiate the shock therapy at a time when thepatient is ready to receive it using the patient activator 100 or analternative patient-initiated trigger. For details regarding a generalpatient activator and other exemplary forms of patient initiatedtriggers, reference is made to the previously incorporated U.S. Pat. No.5,987,356 issued to DeGroot.

Thus, a method and apparatus have been described for delaying thedelivery of a shock therapy in a patient having hemodynamically stable,low rate ventricular tachycardia. Using the methods included in thepresent invention, the incidence of painful shock therapies can beminimized or all together avoided in patients diagnosed with recurring,non-life threatening ventricular tachycardia. By reducing the number ofdelivered shock therapies, the battery longevity of an implantablepacemaker cardioverter defibrillator device is extended. Delaying ascheduled shock therapy can avoid further risk to the patient byallowing the delivered therapy to occur at a time when the patient is ina controlled situation. These benefits may be realized by implementingthe present invention according to the exemplary embodiments disclosedherein. However, it will be understood by one skilled in the art thatvariations or modifications to the described embodiments may be madewithout departing from the scope of the present invention. As such, theexemplary embodiments disclosed herein should not be considered limitingwith regard to the following claims.

1. A method for controlling the delivery of a shock therapy in a cardiacpatient experiencing ventricular tachycardia, comprising: detecting aventricular tachycardia; delivering an anti-tachycardia pacing therapywhen said ventricular tachycardia is detected; determining if theventricular tachycardia is terminated after said anti-tachycardia pacingtherapy is delivered; and delaying a programmed shock therapy based onmeasures of hemodynamic state.
 2. The method of claim 1 wherein saidmeasures of hemodynamic state included setting a hemodynamic thresholdcriteria.
 3. The method according to claim 2 wherein said measure ofhemodynamic threshold state includes setting a hemodynamic stable state.4. The method of claim 1, further comprising: setting a hemodynamicthreshold criteria; detecting a hemodynamic state of the patient; anddelaying said programmed therapy based or comparing said hemodynamicstate of the patient against said hemodynamic threshold criteria.
 5. Themethod of claim 4 wherein said method further comprising: defining acondition for delivering a delayed shock therapy; and delivering saiddelayed shock therapy when the condition for shock delivery issatisfied.
 6. The method of claim 5 wherein said condition fordelivering a delayed shock therapy includes detecting a hemodynamicstate that does not meet said hemodynamic threshold criteria.
 7. Themethod according to claim 5 wherein said condition for delivering adelayed shock therapy includes monitoring a specified amount of elapsedtime.
 8. The method according to claim 1 further comprising: defining acondition for delivering a delayed shock therapy; and delivery saiddelayed shock therapy when said condition for delivering is satisfied.9. The method according to claim 8, further including detecting aposture of the patient.
 10. The method according to claim 9 wherein saidcondition for delivering a shock therapy includes detection of a suddenchange in the patient's position.
 11. The method of claim 8 furtherincluding detecting a sleeping state of the patient.
 12. The method ofclaim 11 wherein a condition for delivering said delayed shock therapyincludes detection of a sleeping state.
 13. The method of claim 8further including detecting myocardial ischemia.
 14. The method of claim8 wherein shock therapy is delivered by the patient or physician.
 15. Adevice-implemented software system for controlling the delivery of ashock therapy in a cardiac patient experiencing ventricular tachycardia,the system comprising: means for detecting a ventricular tachycardia;means for identifying if slow ventricular tachycardia is detected; meansfor delivering programmed anti-tachycardia pacing therapies; means fordetecting termination of the ventricular tachycardia; means forsustained redetection of slow ventricular tachycardia; means forchecking hemodynamic instability; means for delaying programmed shocktherapy; and means for generating patient notification.
 16. The systemof claim 15 wherein said means for detecting termination furthercooperates with means for checking sustained slow ventriculartachycardia.
 17. The system of claim 16 wherein said means for detectinghemodynamic instability cooperates with means for detecting myocardialischemia, means for detecting sleep, means for marking predeterminedtime elapsed and means for confirming if one of shock conditions met andshock conditions not met.
 18. The system of claim 15 wherein said meansfor generating patient notification further includes means for signalinga high alert mode.